This invention relates to powering of portable electronic devices.
Portable electronic devices are normally powered with either a primary or a rechargeable battery. Growth in the portable electronic device market, as well as, changes in usage patterns, has provided opportunities for rechargeable sources of power to power an electronic device. While primary batteries have a greater energy density, their internal resistance is larger, and primary batteries are less suitable in high drain electronic devices. Rechargeable batteries can handle large loads but do not have sufficient energy capacity for many applications.
Fuel cells incorporated into power sources for portable devices promise longer runtimes than conventional battery systems, due to the ability to use high-energy content fuels. Several fuel cell technologies are currently under development for commercialization in portable power applications, such as direct methanol fuel cells (DMFC) and hydrogen polymer electrolyte membrane (PEM) fuel cells.
In a DMFC, the fuel is methanol or mixtures of water and methanol. Methanol or methanol mixtures are delivered as a liquid to an anode chamber in a DMFC, where methanol is oxidized as part of the electrochemical conversion of fuel to electricity. An operational challenge in DMFC systems is “methanol crossover” a phenomenon where at above about 3% methanol concentration in the anode chamber, an unacceptably high amount of methanol migrates across a polymer electrolyte membrane and causes both parasitic losses (reducing runtime) and mixed potentials differences at the cathode causing reduced output power.
Room temperature vapor phase delivery of methanol to the anode of a fuel cell has been proposed. In this approach a passive, gas permeable membrane is placed parallel to and overlapping an anode layer in the fuel cell to convert liquid methanol to a methanol vapor at room temperature. The liquid methanol is provided from a fuel reservoir or fuel cartridge. The approach provides a direct path for methanol delivery to the fuel cell system. The pure methanol vapor supplied to the anode chamber undergoes an in situ dilution with water back-diffusing across the PEM.